In various transmission systems, a weighted and delayed equalizing filter circuit is often employed as one technological measures to electrically compensate for waveform distortions generating due to dispersion or loss in a transmission medium.
In FIG. 19, there is shown a circuit block diagram of a conventional weighted and delayed equalizing filter circuit. Incidentally, the circuit in question is often called as a Transversal Filter, a FIR (Finite impulse Response) digital filter, or a Feed Forward Equalizer. These types of circuits have been described in, for example, non-patent publication 1 (i.e., A. Borjak, et al., “High-Speed Generalized Distributed Amplifier-Based Transversal-Filter Topology for Optical Communication System,” IEEE Trans. Microwave Theory Tech., Vol. 45, No. 8, pp. 1453-1457)
In FIG. 19, reference numeral 1201 designates a filter input terminal; 1202, a filter output terminal; 1203, a load generating in an output circuit connected to output terminal 1202 (it will be hereinafter referred to as an output load); 1204, delay devices or units(T) connected in cascade to the input terminal; 1205, weighting circuits; 1210, an adder which adds therein the outputs from the respective weighting circuits 1205; 1231, an output impedance of the input circuit, which is connected to the input terminal 1201; and 1232, an electric terminating resistance connected to the end of the delay devices connected in cascade.
A signal inputted into the present filter is delayed by a desired delay by respective of the delay devices 1204 to be outputted as respective output signals. Then, desired weight values (filter coefficient) are given by the respective weighting circuits 1205. The signals from these weighting circuits 1205 are added together by the adder 1210 to be eventually outputted via the output terminal 1202.
By applying this kind of signal processing, waveform deformation is compensated for in an electric signal while waveform equalizing is realized in optical communication by carrying out photo-electric conversion of an optical dispersion waveform by a photodiode or the like and by thereafter performing correction.
FIG. 20 exemplifies such a transversal filter circuit. The conventional transversal filter circuit of this type is described in, for example, non-patent publication 2 (“Differential 4-tap and 7-tap Transverse Filters in SiGe for 10 Gb/s Multimode Fiber Optic Equalization”, International Solid-State Circuit Conference (ISSCC) 2003, Session, 10, Paper 10.4, pp. 180-181).
In FIG. 20, reference numeral 1301 designates a filter terminal; 1302, a filter output; 1303, an output load formed in an output circuit, which is connected to the filter output 1302; 1304, delay units connected in cascade to the input side; 1305, weighting circuits; 1306, delay units (T) connected in cascade to the output side; 1307, a load to terminate the cascaded delay units 1307; 1331, an output impedance of the input circuit connected to the input terminal 1301; and 1332, a terminating resistance. In this transversal filter, the output load 1303 forms an adder. By this circuit configuration, an input signal is given delays and weighting values so that desired waveform equalization can be achieved.